The removal of volatile constituents from polymer melts is generally one of the last process steps in the production of polymers. The volatile constituents to be removed may be solvents which were used for the production of the polymers or residues of unreacted monomers after concluded transesterification or esterification or polymerisation reactions and elimination products from transesterification or esterification reactions.
For the purposes of the present invention, volatile constituents are taken to mean any volatile impurities such as monomers, i.e. any starting materials, and volatile components of all kinds such as for example solvents, low molecular weight reaction products, elimination products from the reaction, and decomposition and breakdown products which arise during the reaction, together with any secondary compounds introduced via the feedstocks. In relation to the residual monomers, low molecular weight reaction products are hereinafter taken to mean those with a degree of polycondensation of up to 3.
The removal of such constituents is necessary because such accompanying substances generally give rise to deficiencies in material properties such as thermal stability, processability, flow behavior etc. Such volatile constituents may also give rise to unwanted odor nuisances and/or be harmful to health.
Various apparatuses or processes are known depending on the viscosity of the polymer melts from which the volatile constituents are to be removed. Known apparatuses for this task in polymer melts are, for example, film evaporators or filmtruders, screw machines, strand evaporators or tubular evaporators.
Removal of volatile constituents by chemical means is described, for example, in EP 0 768 337 A1. Removal is effected by addition of CH-acidic organic compounds. The chemical conversion of residual monomers may possibly give rise to products with unwanted environmental impact, which distinctly complicates the use of the products in practical applications. Said process also cannot be used for removing residual solvents.
The process for reducing residual monomers with unsaturated fatty acids according to U.S. Pat. No. 4,215,024 suffers from the same shortcomings.
Another known process describes the reduction of residual monomers by treating the moulding compositions with electron beam radiation, as described in DE 2 843 292 A1. The process is, however, much too costly on a full industrial scale. A process for the removal of residual volatiles by injection of supercritical solvents or gases into the polymer melt with subsequent depressurisation described in EP 0 798 314 A1 has also proved equally costly.
Conventional and usual processes are also based on the removal of volatile constituents by means of mechanically assisted systems. Accordingly, extruders, such as for example in U.S. Pat. No. 4,423,960, DE 2 721 848 C2, EP 0 411 510 B1 or in “Entgasen von Kunststoffen in mehrwelligen Schneckenmaschinen” [Degassing of plastics in multiscrew machines], Kunststoffe 71 (1981), pages 18–26, devolatilising centrifuges (U.S. Pat. No. 4,940,472), friction compaction (EP 0 460 450 A2) or film evaporators (DE 1 925 063 A1 or EP 0 267 025 A1) are used. These processes conventionally have a short residence time of the order of a few minutes.
All the above-stated mechanically assisted processes exhibit the disadvantage that heavy moving parts which move at high rotational speed and rotational frequencies are required in the apparatus. This results in costly apparatus or machinery which is susceptible to malfunctioning and wear. If adequate degassing efficiency is to be achieved, frequent circulation of the product is necessary. At the short residence times, elevated temperatures are, on the one hand, necessary in order to shift the diffusion coefficients and physical equilibria of the volatile components towards favourable values. On the other hand, such elevated temperatures are unavoidable because the frequent circulation of the product results in elevated input of energy. The person skilled in the art is aware that elevated temperatures strongly promote and accelerate unwanted reactions in polymers. Such reactions result in unwanted reductions in quality, such as for example discoloration and/or formation of gel particles, particles or specks and branched structures or even dissociation into monomers. The mechanical energy is usually produced from electrical energy, resulting in higher costs relative to the use of primary energy. Typical energy inputs of such processes are in the order of 0.05 to 0.2 kWh/kg of product.
“Static” degassing processes, which introduce mechanical energy only via pumps, usually gear pumps, are furthermore known to the person skilled in the art. These static processes operate in such a manner that a polymer melt, optionally with additives, is introduced into a degassing vessel, in which the volatile constituents evaporate and are drawn off in gaseous form. Such processes often have multiple stages.
One example of a static process is DE 10 031 766 A1, which describes a two-stage, continuous process for degassing styrene copolymers, in which, in a first stage, the concentration of polymer is adjusted to above 99.8 wt. % in a shell-and-tube heat exchanger with evaporation of volatile constituents and simultaneous input of energy and, in a second stage, the final concentration is obtained in a strand evaporator without intermediate superheating.
A strand evaporator operates by forming free-falling strands of melt in a cabinet, i.e. without supply of mechanical energy. In the cabinet, the strands are generally exposed to a vacuum at elevated temperatures. The heights of such apparatus are limited and thus so too is the residence time during which evaporation may occur. Another disadvantage of the process resides in the very large number of holes which are required for a good degassing result in the strand evaporator. The diameters of the holes are in the lower, single digit millimetre range, while, at throughput of a few tonnes per hour, the holes range in number from some thousands to a hundred thousand. This is disadvantageous. Given the large number of holes, it is to be feared that specks and swollen solids may give rise to blockages and disruption to flow on exit from the hole. Finally, the efficiency of a strand evaporator depends on the stability of the strands, which in turn depends in complex manner on product rheology, flow conditions in the gas space of the strand evaporator, the geometry and quality of the holes and temperature. It is accordingly difficult to establish and control constant processing conditions.
Another example of a static process is U.S. Pat. No. 4,699,976, which describes a two-stage, continuous process for degassing rubber-containing styrene polymers. This process uses two degassing stages which are equipped with shell-and-tube heat exchangers. In the first stage, the polymer solution is concentrated to a residual content of volatile constituents of between 3% and 15%. In the second stage, evaporation is then performed to obtain the desired final concentration. During this process, foaming occurs inside the tubes. This process cannot, however, be used if the concentration of volatile constituents is so low that the polymer melt does not foam because the volume of gas which arises is insufficient.
“Neue Mischverfahren mit geringem Energiebedarf für Polymerherstellung und-aufbereitung” [Novel low-energy mixing processes for polymer production and processing], Chemische Industrie (1985) 37 (7), pages 473 to 476, describes a static process in which, prior to the final stage, an entraining agent is mixed with the polymer before the product is introduced in the final stage into a degassing vessel. As is familiar to the person skilled in the art, the entraining agents used are primarily inert gases, such as for example nitrogen or carbon dioxide or alternatively also water. In EP 0 027 700 A2, an inert entraining agent from the group comprising water, nitrogen, carbon dioxide or hydrocarbons with one to four carbon atoms is mixed with a polymer solution and flashed in a chamber. Both the above-stated processes have disadvantages. Inert gases reduce the performance of the condensers in which the volatile constituents are to be condensed and increase the volume to be conveyed by the vacuum system, so increasing the cost of the process. The use of water is disadvantageous because it entails restricting the temperature of the condensers to above 0° C. in order to prevent freezing and this limits the performance of the condensing system, which must in turn be compensated by a larger and more costly vacuum installation. Water may also react with various polymers, resulting in degradation of molecular weight and impairment of properties.
The concentration of residual volatiles is at thermodynamic equilibrium, when, with ideal mass transfer and after an adequate residence time, the product is at equilibrium with the gas in the stripping apparatus at the selected temperature and the selected pressure, i.e. the concentration undergoes no further change. Changes of a chemical nature due to thermal processes, such as dissociation, decomposition and the like, may severely restrict this statement and are not taken into account in the definition. This definition is familiar to the person skilled in the art. If degassing is to be possible at all in an apparatus, the concentration of residual volatiles in the product must always be higher than corresponds to the thermodynamic equilibrium. A degassing apparatus is particularly advantageous if the concentrations of the constituents to be stripped at the outlet thereof are as close as possible to the thermodynamic equilibrium, it being physically impossible for the concentrations to fall below this level.
Static processes of the above-stated kind have the disadvantage that, for each stage, they permit and enable only one single desgassing step in order to move towards the thermodynamic equilibrium before the product is again discharged from the stage. If, for reasons of degassing efficiency, it is possible in the individual stage to reduce the concentration of a volatile component only by, for example, a factor of 3 relative to the input value, but the degassing task requires a reduction by a factor of 20 relative to the input value, a three-stage installation is required. Obviously, this is costly, highly complex and thus to be avoided if at all possible.
Without exception, the stated apparatuses have short residence times. It is endeavoured to achieve short residence times in order to reduce the products' exposure to elevated temperatures because, as is familiar to the person skilled in the art, exposure to elevated temperatures results in quality impairment, such as for example discoloration and/or in the formation of particles or specks due to secondary and decomposition reactions. The short residence times relate only to those states in which the product has a large surface area per unit mass in the stripping apparatus, i.e. not in “sumps” in which the product is collected prior to discharge from the evaporation apparatus, where the surface area per unit mass is low.
In order to achieve the aim of low content of residual solvents or monomers, in known, mechanically assisted processes and apparatuses temperatures are raised, vigorous surface renewal is achieved, usually by elevated energy input, and it is endeavoured to achieve the best vacuums so that stripping may be performed efficiently, if possible within short residence times. In order to be able to operate with short residence times, the fullest possible use must be made of the effective parameters such as temperature, elevated mechanical energy input for rapid surface renewal and vacuum. The parameters diffusion and mass transfer, which are also highly significant, are product-specific, temperature-dependent physical variables and can only be influenced within this framework.
However, as a consequence of elevated product temperatures, the processes known from the prior art frequently result in partial modification of the products. These modifications may, for example be manifested by discoloration and particle formation due to secondary and decomposition reactions. The formation of particles or also specks entails increased filtration efforts. Filtration units for viscous products are complex and, due to steep pressure gradients, difficult to operate. Temperatures are often increased in order to lower the melt viscosities of the products and so reduce the pressure gradient. However, the increase in temperature in turn has a disadvantageous impact on product quality.